Conventionally, a proximity sensor, a proximity switch, or the like for determining an approach (whether or not within a predetermined distance) of a metal object by the action of an alternating current (AC) magnetic field has been proposed (see Patent Literature 1 to 3, for example).
In a proximity sensor disclosed in Patent Literature 1, two detection coils (sometimes referred to as a reception coil or a search coil) are provided inside a metallic case and perform detection of a position of nonmagnetic metal, magnetic material, or the like. A proximity sensor with high signal to noise (S/N) ratio and high sensitivity can be realized by directly detecting a value of a current difference rather than applying rectangular wave voltages to two detection coils and respectively detecting a current flowing through each of the detection coils for calculating a difference therebetween using a differential circuit or the like. In addition, since respective detection coils have the same magnetic characteristics and electrical characteristics, even when these characteristics change due to temperature change, they change in the same manner respectively, and thus variations due to temperature change cannot easily occur.
For example, in a third embodiment illustrated in FIGS. 10 to 13 of Patent Literature 1, a detection coil L1006 and a reference coil L1008 are accommodated in a cylindrical case 1001a made of austenite-based stainless steel in a state of facing each other, and a logical value indicating whether or not nonmagnetic metal or magnetic metal has approached to within a predetermined distance can be obtained. In other words, the position detection in Patent Literature 1 merely obtains a one-dimensional distance at most.
In a proximity switch disclosed in Patent Literature 2, a nonmagnetic metal body is used as a case 1. Inside the case 1, a detection coil 3 is provided on a detection surface side, and an excitation coil 2 and a detection coil 4 are provided therebehind. The excitation coil 2 is driven at low frequency, and the detection coils 3 and 4 are connected in series in a direction in which induced voltages when an object is not approaching are canceled out each other. Then, the voltage difference is detected by a differential amplification circuit, and magnetic metal is detected by an increase in differential amplification output which is based on the approach of the magnetic metal.
Also in Patent Literature 2, merely a one-dimensional distance to the magnetic is obtained at most.
Also in a non-contact detection device disclosed in Patent Literature 3, two detection coils L1 and L2 are provided. While the first detection coil L1 detects a detection object, the second detection coil L2 is provided for compensating for external electromagnetic waves, and thereby an influence due to the detection object is prevented. That is, two detection coils L1 and L2 are not used together for detecting the detection object.